In the widely performed Laser In-Situ Keratomeleusis (LASIK) procedure, a microkeratome is used to incise the cornea of a patient and create a flap. The flap is then lifted to expose a bed of stromal tissue which is subsequently ablated using an Excimer laser. After ablation, the flap is replaced and allowed to heal. This process, although being somewhat successful in correcting vision deficiencies, has several drawbacks. For example, the creation of a suitable flap for a LASIK procedure is labor intensive and relies heavily on the skill and eye-hand coordination of the surgeon. In addition, the use of a microkeratome often produces an irregular incision which can create vision defects when the irregular, inner flap surface is replaced over a relatively smooth bed of ablated tissue.
As an alternative to corneal reshaping using LASIK, a train of laser pulses having relatively short pulse durations can be directed to a focal point at a predetermined subsurface location within a patient's cornea. This focal point can then be used to photodisrupt tissue at the focal point with precision and accuracy. For example, infrared pulses can be passed through corneal tissue with minimal energy loss to a subsurface focal point. An example of a procedure that uses a pulsed laser beam that is focused to a predetermined, subsurface location within a patient's cornea is disclosed in U.S. Pat. No. 4,907,586, which issued to Bille et al. for an invention entitled “Method for Reshaping the Eye”.
In greater detail, the photodisruption of tissue by a pulsed laser results from a process termed “laser induced optical breakdown” (LIOB). Specifically, in the LIOB process, tissue breakdown occurs in the laser focus due to the extremely high, local electrical field that is generated. This high electric field exceeds the electron binding energy of the tissue atoms, and results in the generation of a microplasma, shockwaves and a cavitation bubble. Typically, the vaporized tissue diffuses out of the cornea within about 30-60 minutes. Importantly, the cavitation bubble created at each focal point collapses under intraocular pressure. As a consequence, this process can be used to effectively reshape the cornea.
When considering the use of subsurface photoablation for corneal reshaping, a general knowledge of the anatomy of the cornea is helpful. In detail, the cornea consists of several layers of tissue which are structurally distinguishable. In order, going in a posterior direction from outside the eye toward the inside of the eye, the various layers of a cornea are: an epithelial layer, Bowman's membrane, the stroma, Descemet's membrane, and an endothelial layer. Of these various structures, the stroma is the most extensive and is generally around four hundred microns thick. For this reason, stromal tissue is generally selected for removal in a refractive correction procedure.
Considering the stroma in further detail, it is generally comprised of around two hundred identifiable and distinguishable layers of lamellae. Each of these layers of lamellae in the stroma is somewhat dome-shaped, like the cornea itself, and they each extend across a circular area having a diameter of about nine millimeters. Each layer includes several lamellae. Unlike the entire layer that a particular lamella is in, each lamella in the layer extends through a shorter distance of only about one tenth of a millimeter (0.1 mm) to one and one half millimeters (1.5 mm). Finally, it is to be noted that, in a direction perpendicular to the layer, each individual lamella is only about two microns thick.
Within the general structure described above, it is to be appreciated that the stroma is considerably anisotropic. Specifically, the strength of tissue within a lamella is approximately fifty times the strength that is provided by the adhesive tissue that holds the layers of lamella together. Due to this relationship between strength and direction in the stroma, it is more efficient to photodisrupt tissue in volumes that extend orthogonally to the lamella layers than it is to photodisrupt tissue in volumes which extend along the lamella layers.
In addition to the considerations described above, another factor that can affect the efficiency and accuracy of a photodisruption procedure is the optical path that the laser takes to reach a focal point at a targeted location. In this regard, it can be appreciated that if the laser must pass through a previously photodisrupted location, the beam can become distorted. This unwanted distortion can affect both the location and size of the focal point and lead to inaccurate results.
In light of the above, it is an object of the present invention to provide devices and methods for photodisrupting stromal volumes having shapes which extend generally normal to the direction of the lamella layers. It is another object of the present invention to provide devices and methods for photodisrupting preselected stromal volumes which avoids placing the surgical laser on a beam path that passes through a previously photodisrupted location to reach a targeted location. Yet another object of the present invention is to provide devices and methods for correcting the refractive properties of a cornea which are easy to use, relatively simple to implement, and comparatively cost effective.